Sputtered spring films with low stress anisotropy

ABSTRACT

Methods are disclosed for fabricating spring structures that minimize helical twisting by reducing or eliminating stress anisotropy in the thin films from which the springs are formed through manipulation of the fabrication process parameters and/or spring material compositions. In one embodiment, isotropic internal stress is achieved by manipulating the fabrication parameters (i.e., temperature, pressure, and electrical bias) during spring material film information to generate the tensile or compressive stress at the saturation point of the spring material. Methods are also disclosed for tuning the saturation point through the use of high temperature or the incorporation of softening metals. In other embodiments, isotropic internal stress is generated through randomized deposition (e.g., pressure homogenization) or directed deposition techniques (e.g., biased sputtering, pulse sputtering, or long throw sputtering). Cluster tools are used to separate the deposition of release and spring materials.

FIELD OF THE INVENTION

This invention relates generally to methods of fabricatingphotolithographically patterned spring structures, and, moreparticularly, to methods of controlling the stress anisotropy during thedeposition of spring films, and to the spring structures formed by thesemethods.

BACKGROUND OF THE INVENTION

U.S. Pat. No. 5,914,218 (Smith et al.) describes photolithographicallypatterned spring structures for use in the production of low cost probecards, to provide electrical connections between integrated circuits, orto form coils that replace surface-mount inductors. A typical springstructure includes a spring finger having an anchor portion secured to asubstrate, and a free portion initially formed on a pad of releasematerial. The spring finger is etched from a thin spring material layer(film) that is fabricated such that its lower portions have a higherinternal compressive stress than its upper portions, thereby producingan internal stress gradient that causes the spring finger to bend awayfrom the substrate when the release material is etched. The internalstress gradient is produced in the thin spring material film either bylayering different materials having the desired stress characteristics,or using a single material by altering the fabrication parameters.

A problem with high-volume production of integrated circuitsincorporating photolithographically patterned spring structures is thatthe released “free” portions of some spring structures fabricatedaccording to conventional methods undergo helical twisting, therebyskewing (displacing) the spring structure tips from their intendedposition. A spring structure is typically designed to curl or bendperpendicular to the underlying substrate (i.e., in a plane passingthrough the spring structure's longitudinal axis) upon release such thatthe tip is located in a predefined position above the substrate. Thetip's position is typically matched to a receiving structure (e.g., acontact pad) formed on an integrated circuit to which the springstructure is electrically connected. Helical twisting causes the springstructure to bend such that the tip is positioned away from thepredefined position, thereby preventing optimal connection between thespring structure and the receiving structure. To make matters worse, theamount of skew tends to vary according to orientation of the springstructure, and spatially over the wafer upon which the spring structuresare produced in high volume. That is, in one region of a wafer, springstructures oriented in a particular direction may experience arelatively small amount of twisting, while spring structures in thatregion oriented in another direction experience pronounced twisting.Also, similarly oriented spring structures that are located in differentregions may experience different amounts of twisting. The amount of skewcan even be zero in certain locations and orientations.

The amount of skew that can be tolerated in a spring structure dependscritically on the application in which the spring structure is used. Forthe manufacture of self-assembling out-of-plane inductors, for example,the specification for the skew is the lesser of +1% of the springdiameter or ±5 microns. For other applications, such as packaging, thespecification may be a little less stringent, and will depend on thesize and spacing of the pads that the springs are designed to contact.

As suggested above, one solution to problems facing high-volumeproduction of integrated circuits incorporating photolithographicallypatterned spring structures is to design systems that take into accountthe expected range of spring structure skew (which would be determinedexperimentally before high-volume production is initiated). However,this solution generates inefficiencies (e.g., wider spring structurespacing and larger contact pads) that increase production costs. Anotherpossible solution would be to identify the locations and orientations onthe wafers at which zero skew occurs in a given fabrication process, andthen only fabricate spring structures in these zero skew locations.However, this solution would limit the wafer area utilized to fabricatespring structures, thereby making high-volume production expensive andcomplicated.

What is needed is a method for fabricating spring structures thatminimizes or eliminates helical twisting, thereby facilitatinghigh-volume production.

SUMMARY OF THE INVENTION

The present invention is directed to methods for fabricating springstructures that minimize helical twisting by reducing or eliminatingstress anisotropy before release, which is characteristic ofconventional spring material films, through manipulation of thefabrication process parameters and/or spring material compositions. Byreducing or eliminating stress anisotropy in the spring material film(i.e., before release), spring structures can be formed at any locationand in any orientation on a substrate without significant helicaltwisting. Accordingly, the complicated and expensive design requirementsof conventional spring structures are eliminated, thereby greatlysimplifying high-volume production and minimizing production costs. Thepresent invention is also directed to the spring structures fabricatedusing these methods.

In accordance with the present invention, spring structures arefabricated such that the spring film includes at least one layer inwhich the internal stress is isotropic (i.e., the internal stressessentially equal in all directions). The present inventors havedetermined that skew is primarily caused by stress anisotropy in thethin film from which the spring structure is formed (i.e., differentstress magnitudes existing along different orthogonal directions withinthe spring material film). By causing the stress in this one or morelayer to be isotropic, total anisotropy in the spring material film isreduced (i.e., provided the remaining layers have the same or lessanisotropy as that typically produced in conventional spring materialfilms). The resulting reduction in stress anisotropy reduces skew(helical twisting) by equalizing stress components along the principalstress axes. The one or more isotropic stress layers may include anisotropic compressive layer, an isotropic tensile layer, an isotropicneutral (zero stress) layer, or any combination thereof. When bothisotropic compressive and isotropic tensile layers are formed, a cappingfilm (e.g., Nickel or Gold) or an intermediate, non-isotropic layer maybe utilized to minimize delamination.

In accordance with an embodiment of the present invention, isotropicinternal stress is generated in one or more layers of a spring materialbefore release by saturating the internal stress of the one or morelayers of the spring material film. Stress saturation causes the one ormore layers to become essentially isotropic (uniform) because furtherapplied stress pushes the spring material beyond its yield point,producing relaxation of the material that relieves the additional stressand causes the internal stress to remain at the saturated level. Byforming at least one layer of the spring material film in this manner,the resulting spring structure exhibits less stress anisotropy than thatproduced using conventional methods, thereby reducing the magnitude ofhelical twisting. Stress saturation of the spring material films isachieved through various methods disclosed herein. In accordance withone disclosed method, stress saturation is achieved by manipulating thefabrication parameters (i.e., temperature, pressure, and electricalbias) formation of the spring material film to generate the saturatedtensile or compressive stress. Methods are also disclosed for tuning thesaturation point of the spring material by varying the depositiontemperature, annealing after growth, formation on silicon and thencooling, and by adjusting the spring material composition tobeneficially modify its saturation characteristics. In one embodiment,the spring material films are balanced (i.e., equal amounts ofcompressive and tensile stress), which produces the thickest springs fora given design radius. In an alternative embodiment, the spring materialfilms are unbalanced (e.g., with a saturated compressive layer that isthicker or thinner than the saturated tensile layer), which producesthinner springs for a given design radius.

In accordance with other disclosed embodiments of the present invention,isotropic internal stress is generated in one or more layers of a springmaterial before release through randomized or directed depositiontechniques. In one embodiment, the compressive anisotropy is reducedthrough randomized deposition caused by gas scattering homogenization.In other disclosed embodiments, anisotropy is reduced through directeddeposition using biased sputtering, pulse sputtering, or long throwsputtering techniques. By controlling the direction in which the springmaterial is deposited using these deposition techniques,stress-anisotropy is reduced or eliminated, thereby increasing springstructure yields and facilitating high-volume production and minimizingproduction costs.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects and advantages of the presentinvention will become better understood with regard to the followingdescription, appended claims, and accompanying drawings, where:

FIGS. 1(A) and 1(B) are top and partial front views, respectively, of aspring material stress gauge illustrating helical twisting caused byanisotropic stress variations in a conventional spring material film;

FIG. 2 is graph showing skew variations relative to biaxial stresscomponents measured using the stress gauge shown in FIGS. 1(A) and 1(B);

FIG. 3 is a top view showing a spring structure according to the presentinvention;

FIG. 4 is a cutaway perspective view taken along line 4—4 of the springstructure shown in FIG. 3;

FIGS. 5(A) and 5(B) are front section views taken along line 5—5 of FIG.3, and show alternative embodiments of spring structures formed inaccordance with the present invention;

FIG. 6 is a graph depicting the stress profile of a balanced springstructure;

FIG. 7 is a graph depicting the stress profile of an unbalanced springstructure;

FIGS. 8(A) through 8(J) are simplified cross-sectional side viewsshowing process steps associated with the fabrication of a springstructure according to several embodiments of the present invention;

FIG. 9 is a graph depicting the effects of annealing on the internalstress of a MoCr alloy;

FIG. 10 is a graph depicting the effects of pressure on internal stressof a MoCr alloy measured at various points in a deposition chamber;

FIGS. 11(A) and 11(B) are simplified side views depicting the depositionof spring material using non-directional and directional depositiontechniques, respectively;

FIG. 12 is a simplified side view showing a deposition chamber utilizedto produce spring structures according to another embodiment of thepresent invention;

FIG. 13 is a simplified top view showing a cluster tool utilized toproduce spring structures in accordance with yet another embodiment ofthe present invention; and

FIG. 14 is a flow diagram showing a process for producing springstructures using the cluster tool of FIG. 13.

DETAILED DESCRIPTION OF THE DRAWINGS

The present inventors have determined that the cause of helical twistingduring high-volume production of spring structures is anisotropic stressin the spring material film. Stress anisotropy is the inequality ofmagnitudes of the biaxial stress along the two orthogonal principalstress axes. The present inventors have also determined that stressanisotropy can occur at any point on a wafer, it can vary spatially overthe wafer, and it can even be zero at certain locations, therebyproducing the variety of helical twisting observed in high-volume springstructure production. In particular, helical twisting occurs-when thelongitudinal axis of a released spring structure is not aligned with oneof the principal stress axes. When an isotropic stress is present, thestresses along the principal stress axes differs, thereby causing thereleased spring finger to twist, skewing the tip from a point that isaligned with the longitudinal axis of the spring structure.

FIGS. 1(A) and 1(B) are top and partial front views, respectively, of aspring material stress gauge (test structure) 10 utilized by the presentinventors to illustrate helical twisting caused by anisotropic stressvariations in a spring material film. Stress gauge 10 includes severalsprings 11-1 through 11-8 that extend in many directions from a centralregion 12. Angular positions (in degrees) are indicated around stressgauge 10 for convenience. According to the selected origin, a baseportion of spring 11-1 is aligned generally along the 0 degree position.

Stress gauge 10 is etched from a conventional spring material film thatis fabricated as described above. Accordingly, stress gauge 10 issubjected to biaxial stress whose principal components are aligned alongtwo orthogonal directions (designated σ_(P1) and σ_(P2)). The effects ofstress anisotropy are quantified by measuring the skew S of a releasedspring, such as spring 11-1 of stress gauge 10. The skew S is defined asthe displacement of the top-most part 11-T of spring 11-1 (see FIG.1(B)) relative to the unreleased material at the base 11-B of spring11-1. The skew S is the result of helical bending in spring 11-1, whichoccurs when (1) the two principal components of the biaxial stresses inthe spring material film from which spring 11-1 is formed are unequal,and (2) the axis X of spring 10-1 is not aligned to either principalcomponent.

FIG. 2 is a table that charts skews measured in the springs of stressgauge 10 (FIG. 1). As FIG. 2 illustrates, by making stress gauge 10 withsprings pointing in many directions, the magnitude and the direction ofthe skew varies in relation to the principal components of biaxialstress. In FIG. 2, the skew is defined as positive when the helicalbending is right handed. As FIG. 2 indicates, for non-zero anisotropy,depending on their orientation, springs can assume either right or lefthanded helical bending, and at four specific orientations aligned withthe principal stress components, the skew is predicted to be zero. Inthe case illustrated in FIG. 2, the zero skew angles are approximately20, 110, 200 and 290 degrees relative to the selected origin. However,as mentioned above, the amount and direction of skew vary widely over awafer, so spring structures formed at different wafer locations aresubjected to a wide variety (i.e., both direction and degree) of helicaltwisting. This wide skew variation makes high-volume productioncomplicated because, in order to avoid helical twisting, each springstructure must be designed to align with the principal stress axes atthe wafer location where that spring structure is fabricated. To furthercomplicate this issue, there is evidence that the principal stresscomponent directions may vary from one layer to another within thespring material film. At a minimum, these design considerations greatlyincrease fabrication costs, and can render high-volume productionimpractical.

FIG. 3 is a plan view showing a spring structure 100 according to anembodiment of the present invention. FIG. 4 is a perspective viewshowing a portion of spring structure 100, and includes a cut-awaysection indicated by section line 4—4 in FIG. 3. Spring structure 100 isformed on a substrate 101, and includes a release material portion 110and a spring finger 120. Substrate 101 (e.g., glass) includes anoptional conductor 105 that can take several forms. Release materialportion 110 is formed above substrate 101 such that it contactsconductor 105 (if present). In one embodiment, release material portion110 may be formed using a metal selected from the group consisting ofTitanium, Copper, Aluminum, Nickel, Zirconium, and Cobalt, or formedusing heavily doped silicon-, to facilitate electrical conductionbetween conductor 105 and spring finger 120. Spring finger 120 includesan anchor portion 122 and a free (i.e., cantilevered) portion 125.Anchor portion 122 is attached to release material portion 110 (i.e.,such that release material portion 110 is located between anchor portion122 and substrate 101). Free portion 125 extends from anchor portion 122over substrate 101.

Similar to prior art spring structures, spring finger 120 is etched froma thin stress-engineered film that is deposited by DC magnetronsputtering or chemical vapor deposition (CVD) techniques, or depositedby plating techniques. In one embodiment, the stress-engineered filmincludes or one or more materials suitable for forming a springstructure (e.g., one or more of molybdenum (Mo), a “moly-chrome” alloy(MoCr), tungsten (W), a titanium-tungsten alloy (Ti:W), chromium (Cr),nickel (Ni), silicon (Si), nitride, oxide, carbide, or diamond. Thedeposition process is performed using gas pressure variations in thedeposition environment during film growth in accordance with knowntechniques (e.g., by varying Argon gas pressure while sputtering thespring material). Typically, this stress-engineered film includes atleast one layer that has a relatively compressive (or less tensile)internal stress, and at least one layer that has a relatively tensile(or less compressive) internal stress, these different stress layersproviding the upward bending bias when the underlying release materialis removed, resulting in a curved, cantilever spring structure. The term“layer” is used herein to describe a cross-sectional region of thestress-engineered film that is formed during a given time period. Forexample, FIG. 5 shows a dashed line generally delineating a first(lower) layer 126 that is formed during a first period of the sputteringprocess, and a second (upper) layer 127 that is formed over (or on)first layer 126 during a subsequent period of the sputtering process.

In accordance with the present invention, spring structure 100 isfabricated in a manner that produces at least one layer (i.e., firstlayer 126 and/or second layer 127) of spring finger 120 having isotropicinternal stress before being released from substrate 101. As utilizedherein, the term “isotropic internal stress” means that the magnitude ofinternal stress measured along both principal stress axes of thestress-engineered film, whether compressive or tensile, is essentiallythe same (i.e., within 1% or less). By definition, forming one or morelayers of spring finger 120 with isotropic stress produces an overallstress profile that is less anisotropic than in conventional springstructures. As pointed out above, anisotropic stress distributions are amajor cause of helical twisting and spring tip skew. By forming at leastone of the compressive layer or the tensile layer with isotropicinternal stress, the total stress exerted on spring finger 120 uponrelease is typically less than in spring structures having anisotropicstresses (i.e., formed using conventional methods), thereby reducing oreliminating helical twisting. Further, the reduction of helical twistingby incorporating one or more isotropic stress layers facilitateshigh-volume production when the isotropic layer is extended over theentire wafer. Note that the relaxation of internal stress occurring infree portion 125 of spring structure 120 after release eliminates thepre-release stress isotropy. However, anchor portion 122, which is notreleased, retains the one or more layers having isotropic internalstress.

In accordance with an aspect of the present invention, the one or moreisotropic, stress layers may include an isotropic compressive layer, anisotropic tensile layer, or both isotropic compressive and tensilelayers. For example, referring to FIG. 4, in accordance with onealternative embodiment, first layer 126 of spring structure 100 has anisotropic compressive internal stress, and non-isotropic tensile secondlayer 127 is formed on first layer 126. In another alternativeembodiment, first layer 126 of spring structure 100 is formed withnon-isotropic compressive stress, and an isotropic tensile second layer127 is formed thereon. In yet another alternative, both the compressiveinternal stress of first layer 126 and the tensile internal stress ofsecond layer 127 are isotropic. Of course, anisotropy is minimized whenspring finger 120 is entirely formed from isotropic material. However,as mentioned above, some applications that require less stringent designspecifications, may be suitably produced from spring fingers that areonly partially isotropic.

In accordance with another aspect, when a spring structure includes bothan isotropic compressive layer and an isotropic tensile layer, one ormore optional structures may be utilized to prevent delamination of thespring finger. Referring to FIG. 4, spring fingers 120 includingbi-level stress (i.e., only compressive layer 126 and tensile layer 127)can have a larger moment than spring fingers that have more than twostress layers. Therefore, for a given design radius, bi-level springfinger 120 may be more likely to delaminate (i.e., separate from releasematerial portion 110 or substrate 101).

FIGS. 5(A) and 5(B) are cross-sectional side views showing springstructures 100A and 100B according to alternative embodiments of thepresent invention, and are taken along section line 5—5 of FIG. 3. Forconvenience, elements of alternative spring structures 100A and 100Bthat are similar to those of spring structure 100 (discussed above) areidentified with the same reference numbers.

Referring to FIG. 5(A), spring structure 100A includes a spring finger120A that is formed with an intermediate layer 128 sandwiched between alower isotropic compressive layer 126A and an upper isotropic tensilelayer 127A. Intermediate layer 128 is formed with an internal stressthat is between the isotropic compressive internal stress of layer 126and the tensile internal stress of layer 127 to reduce the stressgradient between compressive layer 126A and tensile layer 127A. Inalternative embodiments, intermediate layer 128 may include two or moreinternal stress magnitudes, or may be represent a region whose internalstress gradually changes from compressive to tensile. Note that theintermediate stress of layer 128 may not be isotropic, but, whendesigned properly for a given application, isotropic layers 126 and 128can be fabricated to substantially overcome any twisting forces exertedby anisotropy of intermediate layer 128.

Referring to FIG. 5(B), spring structure 100B includes a spring finger120 that is similar to that described above (i.e., isotropic tensilestress layer 127 formed directly on isotropic compressive layer 126). Inaddition, to prevent delamination, spring structure 100B includes acoating 130 (e.g., a plated metal such as Gold or Nickel) that is formedon anchor portion 122 of spring finger 120 either before or after freeportion 125 is released (i.e., after release material located under-freeportion 125 is removed). When coating layer 130 is a plated metal formedafter free portion 125 is released, coating 130 may be deposited on theexposed surface of both upper layer 127 and lower layer 126 of freeportion 125, thereby providing structural and electrical characteristicsthat are superior to spring structures that are formed without platedmetal, or having plated metal formed only on one side. A plated metalcoating 130 (FIG. 5(B)) provides several other potentially importantbenefits to spring structure 100B. For example, plated metal may be usedto electroform the closure of mechanically contacted elements (e.g., anout-of-plane inductor formed using a series of spring fingers bent suchthat the free end of each spring finger contacts the anchor portion ofan adjacent spring finger). Plated metal may also be used to passivatespring finger 120, which is important because most springy metals, suchas stress-engineered metal film 220, form surface oxides. Plated metalmay also be added to increase wear resistance and lubricity. Platedmetal can also provide a compression stop to limit spring compression.Moreover, plated metal may be added to strengthen spring structure 100by adding ductility. Finally, plated metal may be added to blunt theradii of process features and defects that can arise on spring finger120. Note that adding plated metal (coating) 130 over free portion 125will increase the spring constant of spring finger 120 by stiffeningfree portion 125. The above-mentioned benefits are not intended to beexhaustive.

Note that optional conductor 105 is included to provide electricalcoupling of spring structure 100 to an external electrical system (notshown). Note also that the electrical coupling between spring finger 120and conductor 105 necessitates using an electrically conductive releasematerial to form release material portion 110. However, electricalcoupling can also be provided directly to spring finger 120 by otherstructures (e.g., wire bonding), thereby allowing the use ofnon-conducting release materials.

Several methods will now be described for generating the isotropicspring material films utilized in accordance with the present invention.

In accordance with an embodiment of the present invention, a springstructure includes at least one layer having an internal stress that iseither at the compressive saturation point or the tensile saturationpoint of the spring material from which the spring structure is made.The term “saturation point” in this context means a maximum value thatthe internal stress of the spring material (i.e., the spring materialfilm) cannot exceed. Stress saturation causes the spring material tobecome essentially isotropic (uniform) because further applied stresspushes the spring material beyond its yield point, producing relaxationof the material that relieves the additional stress and causes theinternal stress to remain at the saturated level. In particular, if thematerial is stressed (i.e., pushed) beyond its yield point, then thematerial is going to flow or otherwise rearrange its internal structureto relieve the excess stress, returning the material to the so-calledyield point of the material. In other words, no matter how the stress isput in, if a compressive stress greater than the compressive saturationpoint is put into the spring material, then the spring material is goingto relax back to the level of stress that is within the strength limitof the material that is grown (i.e., to the compressive saturationpoint). Similarly, if a tensile stress greater than the tensilesaturation point is put into the spring material, then the springmaterial is going to relax back to the tensile saturation point.Therefore, by saturating the stress, the material is subjected to allthe stress that the material can bear, and beyond the saturation pointthe internal structure no longer responds structurally to store theexcess stress, and it undergoes plastic flow to accommodate additionalstrain imposed upon it.

The present inventors have observed that the compressive stressanisotropy in a spring material film tends to become smaller when thespring material has an internal stress that is at the compressive ortensile saturation point of the spring material. In particular, thepresent inventors formed spring gauges (similar to those describedabove) on several wafers utilizing the methods described below thatproduced saturated internal stress in at least one layer, and thenmeasured the stress along X- and Y-axes on each wafer. The differencesin stress levels were also measured at different pallet locations (eachpallet of the test equipment held more than one wafer; note that otherdeposition tools may hold only one wafer, but the wafers referred towere not processed on a cluster tool). What the inventors observed wasthat anisotropy over the entire wafer (and pallet) was substantiallyreduced when the spring material film included a saturated compressivestress layer.

By forming at least one layer of the spring material film at thesaturation point, the resulting spring structure exhibits less stressanisotropy than that produced using conventional fabrication methods,thereby reducing the magnitude of helical twisting. That is,conventional techniques do not saturate the stress beyond the yieldpoint of the material, so that the spring material film capturesdifferent stresses in different orthogonal directions; and, because thespring material film is below the yield point, these different stressesare captured or frozen into the molecular structure of the film, therebycreating anisotropy. In contrast, because the spring material films ofthe present embodiment are formed using much larger stresses that areclose to the yield point of the spring material, then when stress isapplied in one direction that tends to produce a larger stress in thatdirection than in another direction, the material “refuses” to store theadditional stress and relaxes in the applied direction to the yieldpoint of the material. The anisotropy reduction makes sense from thestandpoint of there being no margin left for anisotropy if the materialis at its yield point. Note that the bulk yield point and the yieldpoint of the growing film are not necessarily the same because the bulkand surface relaxation mechanisms can differ.

Ideally, anisotropy is minimized by fabricating a spring structurehaving a stress profile with only two levels, one formed by acompressive stress layer, and one formed by a tensile stress layer, bothof which being grown at or near the stress saturation points of thespring material. For example, referring again to FIG. 4, in such anideal embodiment, lower layer 126 is formed with internal stress at thecompressive saturated stress level, and upper layer 127 is depositeddirectly on lower layer 126 and formed with internal stress at thetensile saturated stress level. By forming both layers 126 and 127 atthese saturation points and omitting any non-saturated layers, an idealspring structure is formed in which anisotropy is effectivelyeliminated. However, some applications may not require completeelimination of anisotropy (i.e., in applications that can tolerate alimited amount of skew), thereby facilitating the fabrication of springstructures including one or more layers that are non-saturated. Forexample, referring again to FIG. 4, in one alternative embodiment firstlayer 126 of spring structure 100 is formed at the compressivesaturation point, and an isotropic tensile second layer 127 is formedbelow the tensile saturation point. In yet another alternativeembodiment, first layer 126 of spring structure 100 is formed below thecompressive saturation point, and second layer 127 is formed at thetensile saturation point. In yet other alternative embodiments, a springstructure may include saturated layers 126 and 127 separated by anon-saturated layer 128, such as that shown in FIG. 5(A), or saturatedlayers 126 and 127 with a non-saturated layer 130 formed thereon.

As mentioned above, anisotropy is minimized when a spring finger isentirely formed from isotropic material, such as in the embodiment shownin FIG. 4 where both first layer 126 and second layer 127 are atrespective saturation points. The unfortunate side effect of fixing theendpoints of the stress profile at the saturation points is that theother variables remaining for tuning the spring radius are the springlayer thicknesses. This can lead to spring material film thicknessesthat are not optimal. For example, FIG. 6 is a stress profile for abalanced spring structure formed from MoCr alloy (85/15 atomic-%) havinga spring diameter of approximately 500 microns. As indicated in FIG. 6,the saturation point for such a spring material at room temperature isaround −2 and +1 GPa. These saturation points require a spring materialfilm having a thickness of approximately 2.8 microns (i.e., acompressive layer of approximately 0.923 microns and a tensile layer ofapproximately 1.847 microns or more) to produce balanced (i.e.,substantially zero net) total internal stress, and to support the 3 GPastress gradient at the interface between the compressive and tensilelayers. This is many times thicker than many applications would require,and would therefore make such springs much more expensive to produce. Inaddition, because of the large peeling moment (0.92 GPa-μm) at theinterface between layer 126 and release material portion 110 produced bythis arrangement, delamination of spring finger 120 can become aproblem. Note that, in FIG. 6, pre-release stress levels are shown as asolid thick line, and post-release stress levels are indicated with thethinner, dashed line.

One alternative to producing thick balanced spring material films, suchas those consistent with the stress profile shown in FIG. 6, is toproduce unbalanced spring structures. Unbalanced springs havesignificantly non-zero net stress, and, as a result, must be morecarefully designed to avoid failure. FIG. 7 is a stress profile for anunbalanced spring structure formed from MoCr alloy (85/15 atomic-%) andhaving a spring diameter of approximately 500 microns. In the springstructure represented by the stress profile of FIG. 7, the compressivelayer has a thickness of approximately 0.4 microns, and the tensilelayer has a thickness of approximately 1.6 microns. Notice that theunbalanced spring represented by FIG. 7 is about 70% as thick as thebalanced spring in FIG. 6, even though both are formed from the samematerial, thereby reducing fabrication costs over balanced springs(discussed above). However, unlike balanced springs, the net stress inthe unbalanced spring structure is substantially non-zero (e.g., 0.4 GPaor more, as in the example shown in FIG. 7). This substantial non-zeronet stress must be considered in the design of such unbalanced springsin that it produces diameters that are more sensitive to errors instress and film thickness. The peeling moment is also reduced from 0.92GPa-μm (produced by the balanced spring shown in FIG. 6) to about 0.6GPa-μm.

In addition to the balanced and unbalanced spring structures describedabove with reference to FIGS. 6, and 7, which have distinct tensile(negative stress) and compressive (positive stress) layers, it is alsopossible to form a spring using only tensile stress, or only compressivestress, provided that there is a stress differential. For example, aspring structure can be made using a saturated (first) compressivestress layer (i.e., internal stress of −2 GPa) having a thickness of 0.2microns, and an anisotropic (second) compressive stress layer having aninternal stress of −1 GPa and a thickness of 0.57 microns, which isformed on the isotropic layer. Note that in this example the anisotropiccompressive stress layer is relatively tensile with respect to theisotropic compressive stress layer. The resulting spring structure wouldhave a thickness of approximately 0.77 microns, a diameter ofapproximately 413 microns, a net stress after release of 1.29 GPa, and anet peeling moment of 1.02 GPa-μm.

Stress saturation of spring material films is achieved through variousmethods. In accordance with one disclosed method, which is describedwith reference to FIGS. 8(A) through 8(J), stress saturation is achievedby manipulating the fabrication parameters (i.e., temperature, pressure,and RF bias) under which the spring material film is grown to generatethe saturated tensile or compressive stress. Additional methods are alsodescribed below.

Referring to FIG. 8(A), the fabrication method begins with the formationof a conductive release material layer 210 over a glass: (silicon)substrate 101. In one embodiment, release material layer 210 is formedfrom an electrically conductive material, and a portion 210A of releasematerial layer 210 contacts a conductor 105 that is exposed on the uppersurface of substrate 101. In one embodiment, release material layer 210is Titanium (Ti) that is sputter deposited onto substrate 101 to athickness of approximately 0.2 microns or greater. Titanium providesdesirable characteristics as a conductive release material layer due toits plasticity (i.e., its resistance to cracking) and its strongadhesion. Other release materials having the beneficial plasticcharacteristics of titanium may also be used. In other embodiments,release material layer 210 includes another metal, such as Copper (Cu),Aluminum (Al), Nickel (Ni), Zirconium (Zr), or Cobalt (Co). Releasematerial layer 210 may also be formed using heavily doped silicon (Si).Further, two or more release material layers can be sequentiallydeposited to form a multi-layer structure. In yet another possibleembodiment, any of the above-mentioned release materials can besandwiched between two non-release material layers (i.e., materials thatare not removed during the spring finger release process, describedbelow). Alternatively, when it is not necessary to provide electricalconduction between the subsequently deposited spring material layer anda contact pad (such as conductor 105), release material layer 210 can bea non-conducting material such as Silicon Nitride (SiN).

FIGS. 8(B) through 8(D) show the formation and optional annealing of aspring material film in accordance with various embodiments of thepresent invention, discussed below. FIG. 8(B) shows a lower (first)stress-engineered spring material layer 226 formed on release materiallayer 210 using processing techniques selected to produce a compressiveinternal stress in lower layer 226 that is at or near the compressivesaturation point. In one embodiment, spring material layer 226 includesMoCr (85/15-atomic %) sputter deposited to a thickness of 0.1 to 2.5microns using processing parameters suitable for producing saturatedcompressive stress. For example, compressive layer 226 may be generatedusing sputter deposition performed in an Argon atmosphere maintained ata pressure of 0.1 mT or greater, and an applied radio frequency (RF)bias of 0.2 Watt/cm² or greater, the deposition being performed atnominally room temperature (i.e., some heating of the substrate willoccur from the deposition process). In general, this applied bias willresult in additional bombardment of the growing film, and in turn willdrive the material toward the compressive saturation point. Higher biaspower levels can compensate for the effects of gas scattering, andthereby permit operation at higher pressures. FIG. 8(C) shows an upper(second) spring material layer 227 formed on lower layer 226 usingprocessing techniques selected to produce a tensile internal stress inupper layer 227. In another embodiment, the tensile internal stress inupper layer 227 is at or near the tensile saturated stress point. Forexample, MoCr (85/15) spring material is sputter deposited to athickness of 0.1 to 2.5 microns on lower layer 226, depending on thedesign radius and on whether a balanced or unbalanced spring is beingformed. To achieve the desired tensile stress saturation, substrate 101is maintained at room temperature, and sputter deposition is performedin an Argon atmosphere maintained at a pressure of 4 mT or greater, andan applied RF bias in the range of 0 to 0.25 Watts/cm². If acceptablestress saturation is achieved in one or both of lower layer 226 andupper layer 227 using the processing parameters described above,annealing (shown in FIG. 8(D)), may be omitted.

In accordance with another embodiment of the present invention, thedeposition process of FIGS. 8(B) and 8(C) is carried out at an elevatedtemperature that is selected to tune the saturation point of the springmaterial (e.g., MoCr 85/15-atomic % alloy). MoCr alloy is a veryrefractory spring material in that it melts above 2000° C. However, thepresent inventors learned through experimentation that some of thestress in the MoCr alloy may be annealed out ex-situ (e.g., during theanneal process shown in FIG. 8(D)) at fairly low temperatures (i.e.,less than 350° C.). This experimental data is shown in FIG. 9, whichshows the effect of temperature variation on the internal stress of MoCr(85/15) alloy. In FIG. 9, measured data is indicated as diamonds(heating) and squares (cooling), and theoretical data (triangles).Theoretical behavior based on linear elastic response suggests thatformation of saturated spring material film at a high temperature (e.g.,400° C.) would result in a net tensile shift of the stress upon cooling.The actual data suggests that annealing the spring material film and/orforming the spring material film above approximately 100° C. causesrelaxation that results in a tensile shift of the internal stress uponsubsequent cooling. This may be particularly relevant for layers of thespring material film subjected to compressive stress, which can easilybe bombarded in order to reach very high stress levels, but is difficultto make uniform. In other words, because compressive growth is performedat lower pressure, there is less gas scattering present to homogenizethe deposition angle to produce uniform stress. Accordingly, when springmaterial films (e.g., MoCr alloy) are maintained at an intermediatetemperature in-situ during growth (i.e., during one or both of theprocesses shown in FIGS. 8(B) and 8(C)), the saturation point of thespring material is reduced to something that is below its roomtemperature yield. Therefore, growth at elevated temperatures (i.e.,above 100° C., more preferably 150° C. to 400° C., and most preferablyat 350° C.) is believed to achieve isotropic stress characteristics atstress magnitudes that are lower than that achieved during colddeposition. Tuning down the saturation point may have the additionaladvantage of making the material stronger since it will ensure that atroom temperature, the stresses in the material will be far removed fromthe yield point.

The data shown in FIG. 9 also indicates that annealing above 100° C.(e.g., during the process indicated in FIG. 8(D)) may be employed tocause stress relaxation in a spring material film grown at roomtemperature. In particular, annealing in the range of 100 toapproximately 400° C. may produce relaxation, resulting in substantiallyless internal stress upon cooling. Annealing significantly aboveapproximately 350 to 400° C. is not believed to further alter the stressappreciably.

Referring again to FIG. 9, note that as the film is cooled, the MoCrbecomes more tensile due to the relative thermal expansion rate withrespect to silicon. According to another embodiment of the presentinvention, this expansivity difference is exploited to reduce the stresslevel of the compressive layer. In one example, compressive layer 226(FIG. 8(B)) is deposited at elevated temperature (e.g., greater than200° C., preferably 350° C.), and then tensile layer 227 (FIG. 8(C)) isdeposited at a lower deposition temperature (e.g., less than 200° C.,preferably 50° C.). The tensile stress of layer 227 is optionally keptuniform by non-saturating methods, if needed, such as usinghigh-pressure gas scattering (discussed below). During the cooling tothe lower temperature the compressive stress would be reduced by anamount given by the equationΔσ=Y′(α_(MoCr)−α_(Si))(T ₂ −T ₁)where α_(MoC) and α_(Si) are the expansivities of the MoCr film andsilicon substrate, respectively, and T_(1 and T) ₂ are the twotemperatures. Y′ is the biaxial elastic modulus of MoCr.

Referring to FIG. 8(E), after the spring material film is produced usingone or more of the methods described above, elongated spring material(first) masks 230 (e.g., photoresist) are then patterned over a selectedportion of the exposed upper layer 227. Next, as indicated in FIG. 8(F),exposed portions of stress-engineered material film 220 surrounding thespring material mask 230 are etched using one or more etchants 240 toform a spring material island 220-1. Note that this etching process isperformed such that limited etching is performed in portions 210B ofrelease layer 210 that surround spring material island 220-1 such thatat least a partial thickness of release layer portion 210B remains onsubstrate 101 after this etching step. In one embodiment, the etchingstep may be performed using, for example, a wet etching process toremove exposed portions of stress-engineered material film 220. Thisembodiment was successfully performed using cerric ammonium nitratesolution to remove a MoCr spring metal layer. Many additional etchingprocess variations and material selections may be used in place of theexamples given, which are not intended to be limiting.

FIG. 8(G) shows spring material island 220-1 and release material 210after spring material mask 230 (FIG. 8(F)) is removed. Note again thatelectrical connection between conductor 105 and spring material island220-1 is provided through portion 210A of release material layer 210.

Referring to FIG. 8(H), release (second) mask 250 (e.g., photoresist) isthen formed on a first portion 220-1A of spring material island 220-1.Release mask 250 defines a release window RW, which exposes a secondportion 220-1B of spring material island 220-1 and surrounding portions210B of release material layer 210.

Referring to FIG. 8(I), a release etchant 260 (e.g., a buffered oxideetch) is then use to selectively remove a portion of the releasematerial layer from beneath the exposed portion of the spring materialisland to form spring finger 120 (discussed above with reference toFIGS. 1-3). Specifically, removal of the exposed release material causesfree portion 125 to bend away from substrate 101 due to the internalstress variations established during the formation of the springmaterial film (discussed above). Note that anchor portion 122 remainssecured to substrate 101 by release material portion 110, which isprotected by release mask 250. Note also that when release materialportion 110 is formed from a conductive release material, the resultingspring structure is electrically coupled to conductor 10.5.

Finally, FIG. 8(J) shows spring structure 100 during the removal ofrelease mask 250 Referring briefly to FIG. 8(I), note that thenegative-sloped sidewall of release mask 250 produces an exposed edge250-E under an optional post-release coating (e.g., coating 130 in FIG.5(A)). This exposed edge allows access of a solvent that dissolvesrelease mask using known techniques. For example, when the release maskis image-reversed photoresist, acetone can be used as the solvent. Asthe release mask is dissolved, residual coating portions formed thereonare lifted off. If necessary, agitation may be used to accelerate thelift-off process.

The example described above is particularly directed to a MoCr (85-15atomic %) alloy spring material. In accordance with yet another aspectof the present invention, the saturation point of the spring materialmay also tuned by adjusting the spring material composition tobeneficially modify its yield point. Some metals are softer and havelower yield points than Mo and Cr, and therefore the stress of acomposition including these softer metals is typically saturated atlower stress values. Preferably, materials with a ratio of elasticmodulus to yield point that produces spring radii in the range desiredare selected. For example, NiZr alloys have saturated stress at lowerstress values than those of MoCr alloys. Other possibilities include theuse of other Ni alloys, NiCu alloys (e.g., Monel®), BeCu, PhosphorBronze, or alloys of refractory materials such as Mo, Cr, Ta, W, Nb. Inyet another example, the yield point of a MoCr alloy may be tuned(reduced) by including one or more “soft” metals (e.g., In, Ag, Au, Cu).

Even with significant relaxation and/or saturation point tuning, springmaterial films utilizing compressive and tensile saturated stress layersproduce significant stress moments, which can result in delamination ofthe spring structure along spring release layer or releaselayer/substrate interface. It is therefore desirable to deposit springmaterial with isotropic stress at levels continuously (gradually)ranging within the most compressive to the most tensile valuessustainable in the spring material. Continuous variation of the stressis desirable because it reduces the amount of residual stored elasticenergy in the spring after release. In FIG. 6, for example, the residualstress (dashed line) of a saturated stress spring is shown to besubstantial, whereas had the initial stress consisted of a linear ratherthan a stepwise profile, the residual stress (in the bending direction)would be approximately zero. Stepwise profiles with more than two levelsapproximate a linear profile better, and have less residual stress. Suchprofiles also have less abrupt stress transitions. Each transitionproduces internal shear within the spring. The present inventors haveobserved that under some conditions, spring breakage is correlated withthe abruptness of the stress level transitions. One solution to theabrupt stress level transitions associated with saturated stresssprings, similar to that shown in FIG. 5(B), is to include an additionalmetal layer (e.g. gold) that is usually sputtered, not plated, on thetop and/or bottom of the spring material to suppress breakage. However,including this solution increases production costs. It is thereforedesirable to have a process for creating isotropic stress over acontinuous range of stress (i.e., gradually changing from compressive totensile).

In accordance with another embodiment of the present invention,anisotropy is reduced in spring material films through variation inpressure during the deposition process. In particular, by increasing thepressure inside the deposition chamber, the arriving species arescattered by the sputtering gas (typically Ar gas). Experiments using aWilder MV40 modular vertical magnetron deposition tool (a verticalcathode sputter system) showed the anisotropy of a MoCr alloy (85/15)film to fall almost to zero at around 20 mT. FIG. 10 shows stress vs.pressure calibration data for the Wilder tool with a 1-inch substrate totarget spacing. This figure shows that there is a convergence of thestresses measured at three separate pallet positions at the highestpressure (20 mT). In contrast, the stress differences are greatest at 10mT. Suitable anisotropy reduction is achieved above approximately 15 mT.At pressures that are too high (e.g., greater than 20 mT), films grownat low temperature and/or zero bias may tend to become mechanicallyweak, and as a result, the tensile stress decreases. The cause ofmechanical weakness is likely to be large voids in the material. Thepressure at which this weakness occurs is very dependent on the sputterconfiguration and other parameters, such as distance and power. Suitablefilms are formed when the pressure-distance product is in the range of10 to 350 mT-cm. An applied RF bias (e.g., 0.05 Watts/cm² or more) tendsto increase the bombardment of the substrate, thereby increasing thedensity. An elevated substrate temperature (e.g., 150° C. or more) willincrease the surface diffusion of material incorporated into the growingfilm, also tending to increase the density. Heating and/or biasing thesubstrate will help avoid mechanical weakness. Properly balancingpressure, temperature and bias is believed to produce tensile stresshomogeneity through gas scattering while avoiding the mechanicaldegradation of the film due to porosity. Homogenizing the stress via gasscattering has two distinct advantages over material saturation in thatthe effect can potentially operate at any stress setpoint required forthe spring design, and the stress can potentially be variedcontinuously. Although FIG. 10 might make it appear that pressurehomogenization can only work for creating uniform tensile stress, withsufficient applied bias to the substrate, the inventors believe that thespring material may be bombarded into a compressive state at highpressure as well. In accordance with the present embodiment, therefore,a spring structure including both compressive (lower) and tensile(upper) layers and having a continuously varying internal stress profiletherebetween is produced by depositing at a high pressure whilegradually decreasing the applied RF bias during the deposition run inorder to make the stress trend from compressive to tensile.

In accordance with another embodiment of the present invention, anisotropic spring material film is formed using directed depositiontechniques. That is, the present inventors have determined that anotherway to make the internal stress of the spring material film uniform inis to utilize a directional deposition process in which every atom orion that hits the growing film is moving, at least on average, normal tothe substrate.

FIG. 11(A) is a simplified side view showing a typical sputterdeposition process used to form a spring material film 1120 on a wafer(substrate) 1101, upon which is formed a release material layer 1110.According to known methods, a voltage is applied to a target 1130, whichis made of spring material (e.g., MoCr), such that particulate material1135 is separated (sputtered) from target 1130 and travels toward wafer1101. As indicated in FIG. 11(A), the typical sputter deposition processcauses deposition material 1135 to assume a broad angular distributionthat forms spring material film 1120. Accordingly, separate points onwafer 1101 are subjected to differing angular distributions of material,and therefore the principal axes of stress formed in spring materialfilm 1120 are aligned in various directions throughout wafer 1101. In aplanetary system that mounts wafers on a double-rotation mechanism forexample, the principal axes of the stress tend to align with the radialand tangential directions of the wafer. One would expect a similareffect in most cluster tools that utilize rotating magnets behind thesputter source.

In accordance with the method depicted in FIG. 11(B), one or moredirectional deposition methods are utilized to influence the path takenby material 1135 leaving target 1130 such that a majority of the atomsor ions that hit the growing film 1120 are moving normal to substrate1101. The present inventors have experimentally determined that the useof directional deposition greatly reduces stress anisotropy in springmaterial film 1120 because material 1135 strikes all points on wafer1101 in only one direction, thereby eliminating the differing angulardistributions associated with conventional deposition methods. Note thatrelease material layer 1110 is formed in the manner described above withreference to FIG. 8(A), and further processing of the isotropic springmaterial film 1120 is performed as described above with reference toFIGS. 8(E) through 8(J).

According to the present invention, various directional depositionmethods that have been previously used to produce, for example, viastructures in conventional integrated circuit devices are utilized toproduce isotropic spring material films. Such methods include the use ofbiased ionized deposition, long throw sputtering, and collimatedsputtering, each of which is described in more detail below. Thesedirectional deposition methods have been used to fill high aspect ratiovias, which is important for IC (integrated circuit) manufacturing inorder to reduce capacitance, lower resistance and pack more circuitsonto a chip. However, the present inventors do not believe these methodshave been previously considered for producing spring material films, inpart, because the problems addressed in spring material film formationare quite different from those of via formation. That is, in contrast tovia formation in which atoms are directed into a hole, directeddeposition is utilized to minimize internal stress anisotropy during theformation of spring material film. Directed deposition has two distinctadvantages over material saturation in that the effect can potentiallyoperate at any stress setpoint required for the spring design, and thestress can potentially be varied continuously to produce thecompressive-to-tensile stress profile associated with the springmaterial films of the present invention.

In biased ion deposition, the directionality of atoms/ions is influencedusing an applied RF bias. Sputter deposition is usually performed bygenerating a plasma in a deposition chamber containing an Argon gasatmosphere. The Argon gas and the sputtered spring material (e.g.,metals) in a conventional plasma consist primarily of neutrals andpositive ions. Most of the metals in a typical plasma are neutrals, andhence they will not be influenced by the applied bias. Much of the Argonbombardment that produces compressive stress is from reflected Argonneutrals. Utilizing an RF bias of 0.25 Watts/cm² or more to orient thedirection of the bombardment and the depositing flux therefore isrequired for producing more ions. A lot of effort has gone into makingmore ions in the sputter plasma in the context of via formation and thelift-off of sputtered films. For via filling, in addition to an appliedbias (typically a negative self-bias resulting from an RF bias to thesubstrate) effort must be made to ionize the majority (i.e., greaterthan 50%) of the metal species in the plasma. Generating more ions canbe achieved a variety of ways known in the art.

FIG. 12 is a schematic side view showing a system for generating ahighly ionized plasma in a deposition (vacuum) chamber 1200 to produce aspring material film in accordance with an embodiment of the presentinvention. A substrate (wafer) 1201 is placed on a pedestal electrode1205 at a bottom of deposition chamber 1200. Note that substrate 1201already has formed thereon a release material layer 1210, which isformed in the manner described above. Located at an upper end of chamber1200 is a target 1230, and a vertically oriented coil 1240 is wrappedaround the space located between target 1230 and substrate 1201. A DCpower supply 1250 negatively biases target 1230. An RF power source 1253supplies electrical power in the megahertz range to inductive coil 1240.The DC voltage applied between target 1230 and substrate 1201 causes theprocessing gas supplied to the chamber to discharge and form a plasma.The RF coil power inductively coupled into chamber 1200 by coil 1240increases the density of the plasma. Magnets 1260 disposed above target1240 significantly increase the density of the plasma adjacent to target1230 in order to increase the sputtering efficiency. Another RF powersource 1257 applies electrical power in the frequency range of 100 kHzto a few megahertz to pedestal 1205 in order to bias it with respect tothe plasma. This technique is described in S. M. Rossnagel and J.Hopwood, “Metal Ion Deposition from Ionized Magnetron SputteringDischarge”, J. Vac Sci Tech, B 12(1), pp 449-453; see also U.S. Pat. No.6,238,533 to Satitunwaycha et al. The resulting deposition of sputteredMoCr material from target 1230 is acted upon by the RF bias such that amajority of the atoms or ions that hit the film growing on releasematerial 1210 are moving normal to substrate 1201.

Pulsed sputtering is another method for producing more ions to formisotropic spring material films in accordance with the presentinvention. High current pulsed sputtering is a way to make lots of ions.There have been some recent developments in this area lead by UlfHelmersson in Linköping Sweden. These developments use pulses ofhundreds of amps, and megawatts of power, that last for 10's or 100's ofmicroseconds (see Gudmundsson, J. T.; Alami, J.; Helmersson, U.,“Evolution of the Electron Energy Distribution and Plasma Parameters ina Pulsed Magnetron Discharge”, Applied Physics Letters, 78(22), pp3427-9 (2001); see also Helmersson, U.; Khan, Z. S.; Alami, J.,“Ionized-PVD by Pulsed Sputtering of Ta for Metallization ofHigh-Aspect-Ratio Structures in VLSI”, Proceedings of InternationalConference on Advanced Semiconductor Devices and Microsystems (ASDAM),pp 191-5 (2000)). This results in a much denser plasma than what isachieved under continuous operating conditions. A big advantage of thistechnique is that it does not require placing an RF coil in the processchamber. The degree of ionization is close to 100%, which allows the useof applied electric or magnetic fields to control the direction of thespring material striking the substrate.

According to another specific embodiment, spring materials are grownusing “long throw” techniques in which a wafer (substrate) is placed onediameter of the wafer or more away from the target (i.e., when the waferis stationary). The farther away the sputter source, the more collimatedthe deposited material becomes. This approach to making sputter systemshas also been used for filling vias. Typically, the distance between thetarget and the substrate in a cluster tool in about 2 inches (less thanone wafer diameter). If however the distance is increased to one waferdiameter or more, the range of angles with which the flux arrives isreduced. By cutting out shallowest angles of the deposition, the stressanisotropy is believed to be reduced. Note that, if the wafer(substrate) is moving, or multiple substrates are used, to achieve theeffects of long throw, the distance must be on the order of the size ofthe holder containing the substrates, and its range of motion, if any,relative to the target.

The present inventors have looked into various deposition geometries andwafer handling configurations. In single chamber systems with multipletargets, cross contamination has produced some unexpected results. Theseinclude metal contamination altering the stress level in the film for agiven process condition, electrochemical effects during etchingresulting from interface intermixing, and the presence of residues afteretching due to insoluble contaminants. All of the effects occur becausesingle chamber systems have sputter cathodes for materials that form therelease, spring and cladding layers of the spring. When one cathode issputtering and the other is exposed, the exposed cathode accumulatescontamination build-up on its target.

In accordance with another embodiment of the present invention, springstructures are produced using an integrated multi-chamber tool (“clustertool”) that is configured such that separate chambers are utilized foreach of the sputter targets and the etching function, therebymaintaining the integrity of the sputter targets throughout theproduction process.

FIG. 13 is a simplified top plan view showing a cluster tool 1300configured in accordance with an embodiment of the present invention.One such cluster tool is the Endura RTM 5500 platform, which isfunctionally described by Tepman et al. in U.S. Pat. No. 5,186,718.

Wafers 1301 are loaded into the system by two independently operatedloadlock chambers 1305 configured to transfer wafers into and out of thesystem from wafer cassettes loaded into the respective loadlockchambers. The pressure of a first wafer transfer chamber 1304 to whichthe loadlocks can be selectively connected via unillustrated slit valvescan be regulated. After pump down of the first transfer chamber 1304 andof the selected loadlock chamber 1303, a first robot 1306 located in thefirst transfer chamber 1304 transfers the wafer from the cassette to oneof two wafer orienters 1308 and then to a degassing orienting chamber1312. First robot 1306 then passes the wafer into an intermediatelyplaced plasma preclean chamber 1314, from which a second robot 1316transfers it to a second transfer chamber 1318, which is kept at a lowpressure. Second robot 1316 selectively transfers wafers to and fromreaction chambers arranged around its periphery. When materials withinthe substrate have a tendency to retain moisture, such as polyimide, itis common to include a dehydration chamber on the sputter tool to bakeaway the retained moisture prior to processing. This step usuallyprecedes the preclean step.

In accordance with the present embodiment, a first deposition chamber1320 is utilized to form a release material layer on each substrateaccording to the methods described above, one or more second depositionchambers 1322 and 1324 are used to form spring material film on therelease material layer. In particular, first deposition chamber isconfigured with a suitable target formed from a selected releasematerial (e.g., Titanium), and second deposition chambers 1322 and/or1324 are configured with suitable targets formed from a selected springmaterial (e.g., MoCr alloy). Accordingly, as indicated in the flowdiagram shown in FIG. 14, each wafer 1301 is moved into first chamber1320 (block 1410) for release layer formation (block 1420), and thenmoved to second chamber 1322/1324 (block 1430) for spring material filmformation (block 1440) according to one or more of the methods describedabove. Note that, when two different process parameters are utilized toproduce the compressive layer and tensile layer, both chambers 1322 and1324 are configured with similar spring material targets and otherwiseoptimized for the respective different processes. For example, if thecompressive layer and tensile layer are formed at differenttemperatures, then chambers 1322 and 1324 are maintained at thesedifferent temperatures to minimize the time required to change betweenthese two temperatures. After formation of the spring material film,substrates 1301 are removed from the cluster tool, and transferred to asuitable third chamber (block 1450 in FIG. 14) for subsequent etchingand spring finger formation (block 1460) according to the processdescribed above. By performing these deposition processes in separatechambers, and by performing etching/release in yet another chamber (ormore), cross contamination of the respective targets is avoided, therebyavoiding the problems associated with single chamber production.

In yet another embodiment, a multi-chamber sputter tool with chambersarranged in a vertical inline geometry and configured as described abovecan be used in place of cluster tool 1300.

Although the present invention has been described with respect tocertain specific embodiments, it will be clear to those skilled in theart that the inventive features of the present invention are applicableto other embodiments as well, all of which are intended to fall withinthe scope of the present invention.

1. A spring structure comprising: a substrate having an upper surfacedefining a first plane; and a spring finger formed from astress-engineered film deposited over the substrate, the spring fingerhaving an anchor portion attached to the substrate and defining a secondplane that is parallel the first plane, the spring finger also having acurved free portion extending from the anchor portion and bending out ofthe second plane away from the substrate, wherein the stress-engineeredfilm forming the anchor portion of the spring finger includes at leastone layer having an isotropic internal stress.
 2. The spring structureaccording to claim 1, wherein the anchor portion of the spring fingercomprises a first layer having a compressive internal stress, and asecond layer formed over the first layer having a tensile internalstress.
 3. The spring structure according to claim 2, wherein thecompressive internal stress of the first layer is isotropic.
 4. Thespring structure according to claim 2, wherein the tensile internalstress of the second layer is isotropic.
 5. The spring structureaccording to claim 2, wherein both the compressive internal stress ofthe first layer and the tensile internal stress of the second layer areisotropic.
 6. The spring structure according to claim 5, furthercomprising an intermediate layer formed between the first layer and thesecond layer, wherein an internal stress of the intermediate layer has amagnitude that is between the isotropic compressive internal stress ofthe first layer and the isotropic internal tensile stress of the secondlayer.
 7. The spring structure according to claim 5, further comprisinga third layer formed on at least one of the first layer and the secondlayer, wherein an internal stress of the third layer has a magnitudethat is between the isotropic compressive internal stress of the firstlayer and the isotropic internal tensile stress of the second layer. 8.The spring structure according to claim 7, wherein the third layercomprises a metal selected from Gold (Au) and Nickel (Ni).
 9. The springstructure according to claim 1, wherein the spring finger comprises oneor more materials selected from molybdenum (Mo), tungsten (W), titanium(Ti), chromium (Cr), nickel (Ni), silicon (Si), silicon oxide (SiOx),silicon nitride (SiNx), carbide, and diamond.
 10. The spring structureaccording to claim 9, further comprising at least one element selectedfrom the group of Copper (Cu), Gold (Au), Silver (Ag), and Indium (In)in an amount between 0 and 25% by weight.
 11. The spring structureaccording to claim 1, further comprising a release material portionlocated between the anchor portion of the spring finger and thesubstrate, wherein the release material portion is electricallyconductive.
 12. The spring structure according to claim 11, wherein therelease material portion comprises at least one metal selected from thegroup consisting of Ti, Cu, Al, Ni, Zr, and Co.
 13. The spring structureaccording to claim 11, wherein the release material portion comprisesheavily doped silicon.
 14. A spring structure comprising: a substratehaving an upper surface defining a first plane; and a spring fingerformed from a stress-engineered film deposited over the substrate, thespring finger having an anchor portion attached to the substrate anddefining a second plane that is parallel the first plane, the springfinger also having a curved free portion extending from the anchorportion and bending out of the second plane away from the substrate,wherein the spring finger is formed from a spring material defining acompressive saturation point and a tensile saturation point, and whereinthe anchor portion of the spring finger includes at least one layerhaving an internal stress that is at one of the compressive saturationpoint and the tensile saturation point.
 15. The spring structureaccording to claim 14, wherein the spring finger comprises a first layerhaving a compressive internal stress, and a second layer formed over thefirst layer having a tensile internal stress.
 16. The spring structureaccording to claim 15, wherein the compressive internal stress of thefirst layer is at the compressive saturation point.
 17. The springstructure according to claim 15, wherein the tensile internal stress ofthe second layer is at the tensile saturation point.
 18. The springstructure according to claim 15, wherein the compressive internal stressof the first layer is at the compressive saturation point and thetensile internal stress of the second layer is at the tensile saturationpoint.
 19. The spring structure according to claim 18, furthercomprising an intermediate layer formed between the first layer and thesecond layer, wherein an internal stress of the intermediate layer has amagnitude that is between the compressive saturation point and thetensile saturation point.
 20. The spring structure according to claim18, further comprising a third layer formed on at least one of the firstlayer and the second layer, wherein an internal stress of the thirdlayer has a magnitude that is between the compressive saturation pointand the tensile saturation point.
 21. The spring structure according toclaim 18, further comprising a third layer formed on the second layer,wherein the third layer comprises a metal selected from Gold (Au) andNickel (Ni).
 22. The spring structure according to claim 18, wherein anet moment of the first layer has a magnitude that is substantiallyequal to a net moment of the second layer, such that a net totalinternal stress of the spring finger is substantially zero.
 23. Thespring structure according to claim 18, wherein a net moment of thefirst layer has a magnitude that is substantially different from a netmoment of the second layer, such that a net total internal stress of thespring finger is substantially non-zero.
 24. The spring structureaccording to claim 14, wherein the spring finger comprises one or morematerials selected from molybdenum (Mo), tungsten (W), titanium (Ti),chromium (Cr), nickel (Ni), silicon (Si), silicon oxide (SiOx), siliconnitride (SiNx), carbide, and diamond.
 25. The spring structure accordingto claim 24, further comprising at least one element selected fromcopper (Cu), gold (Au), silver (Ag), and indium (In) in an amountbetween 0 and 25% by weight.
 26. The spring structure according to claim14, wherein the spring material comprises nickel (Ni) and zirconium(Zr).
 27. The spring structure according to claim 14, wherein the springmaterial comprises an alloy comprising nickel (Ni).
 28. The springstructure according to claim 27, wherein the alloy further comprisescopper (Cu).
 29. The spring structure according to claim 14, wherein thespring material comprises beryllium (Be) and copper (Cu).
 30. The springstructure according to claim 14, wherein the spring material comprisesan alloy comprising copper (Cu), tin (Sn) and phosphorus (P).
 31. Thespring structure according to claim 14, wherein the spring materialcomprises an alloy of at least one of the group consisting of Molybdenum(Mo), Chromium (Cr), Tantalum (Ta), Tungsten (W), and Niobium (Nb). 32.An array of spring structures comprising: a substrate having an uppersurface; and a plurality of spring fingers formed from astress-engineered film deposited over the substrate, each spring fingerhaving an anchor portion attached to the substrate, said each springfinger also having a curved free portion extending from the anchorportion, wherein the stress-engineered film forming the anchor portionof each spring finger includes at least one layer having an isotropicinternal stress.
 33. The spring structure according to claim 32, whereinthe anchor portion of the spring finger comprises a first layer having acompressive internal stress, and a second layer formed over the firstlayer having a tensile internal stress.
 34. The spring structureaccording to claim 33, wherein the compressive internal stress of thefirst layer is isotropic.
 35. The spring structure according to claim33, wherein the tensile internal stress of the second layer isisotropic.
 36. The spring structure according to claim 33, wherein boththe compressive internal stress of the first layer and the tensileinternal stress of the second layer are isotropic.
 37. The springstructure according to claim 36, further comprising an intermediatelayer formed between the first layer and the second layer, wherein aninternal stress of the intermediate layer has a magnitude that isbetween the isotropic compressive internal stress of the first layer andthe isotropic internal tensile stress of the second layer.
 38. Thespring structure according to claim 32, wherein the spring fingercomprises one or more materials selected from molybdenum (Mo), tungsten(W), titanium (Ti), chromium (Cr), nickel (Ni), silicon (Si), siliconoxide (SiOx), silicon nitride (SiNx), carbide, and diamond.
 39. Thespring structure according to claim 38, further comprising at least oneelement selected from the group of Copper (Cu), Gold (Au), Silver (Ag),and Indium (In) in an amount between 0 and 25% by weight.
 40. The springstructure according to claim 32, further comprising a release materialportion located between the anchor portion of the spring finger and thesubstrate, wherein the release material portion is electricallyconductive.
 41. The spring structure according to claim 40, wherein therelease material portion comprises at least one metal selected from thegroup consisting of Ti, Cu, Al, Ni, Zr, and Co.
 42. The spring structureaccording to claim 41, wherein the release material portion comprisesheavily doped silicon.
 43. An array of spring structures comprising: asubstrate having an upper surface; and a plurality of spring fingersformed from a stress-engineered film deposited over the substrate, eachspring finger having an anchor portion attached to the substrate, saideach spring finger also having a curved free portion extending from theanchor portion, wherein each spring finger is formed from a springmaterial defining a compressive saturation point and a tensilesaturation point, and wherein the anchor portion of each spring fingerincludes at least one layer having an internal stress that is at one ofthe compressive saturation point and the tensile saturation point. 44.The spring structure according to claim 43, wherein the spring fingercomprises a first layer having a compressive internal stress, and asecond layer formed over the first layer having a tensile internalstress.
 45. The spring structure according to claim 44, wherein thecompressive internal stress of the first layer is at the compressivesaturation point.
 46. The spring structure according to claim 44,wherein the tensile internal stress of the second layer is at thetensile saturation point.
 47. The spring structure according to claim44, wherein the compressive internal stress of the first layer is at thecompressive saturation point and the tensile internal stress of thesecond layer is at the tensile saturation point.
 48. The springstructure according to claim 47, further comprising an intermediatelayer formed between the first layer and the second layer, wherein aninternal stress of the intermediate layer has a magnitude that isbetween the compressive saturation point and the tensile saturationpoint.
 49. The spring structure according to claim 47, wherein a netmoment of the first layer has a magnitude that is substantially equal toa net moment of the second layer, such that th a net total internalstress of the spring finger is substantially zero.
 50. The springstructure according to claim 47, wherein a net moment of the first layerhas a magnitude that is substantially different from a net moment of thesecond layer, such that th a net total internal stress of the springfinger is substantially non-zero.
 51. The spring structure according toclaim 43, wherein the spring finger comprises one or more materialsselected from molybdenum (Mo), tungsten (W), titanium (Ti), chromium(Cr), nickel (Ni), silicon (Si), silicon oxide (SiOx), silicon nitride(SiNx), carbide, and diamond.
 52. The spring structure according toclaim 51, further comprising at least one element selected from copper(Cu), gold (Au), silver (Ag), and indium (In) in an amount between 0 and25% by weight.
 53. The spring structure according to claim 43, whereinthe spring material comprises nickel (Ni) and zirconium (Zr).
 54. Thespring structure according to claim 43, wherein the spring materialcomprises an alloy comprising nickel (Ni).
 55. The spring structureaccording to claim 54, wherein the alloy further comprises copper (Cu).56. The spring structure according to claim 43, wherein the springmaterial comprises beryllium (Be) and copper (Cu).
 57. The springstructure according to claim 43, wherein the spring material comprisesan alloy comprising copper (Cu), tin (Sn) and phosphorus (P).
 58. Thespring structure according to claim 43, wherein the spring materialcomprises an alloy of at least one of the group consisting of Molybdenum(Mo), Chromium (Cr), Tantalum (Ta), Tungsten (w), and Niobium (Nb).